1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus that controls the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine, and in particular relates to an air-fuel control apparatus that carries out the detection of malfunctions of an air-fuel ratio detection sensor that detects the air-fuel ratio in the exhaust of the internal combustion engine.
2. Background Art
Conventionally, as a method for detecting damage to an air-fuel ratio sensor (hereinbelow, referred to as the LAF sensor) that is provided in the exhaust system of the engine and outputs a signal roughly proportional to the oxygen concentration in the exhaust, the change in the output properties of the LAF sensor are monitored and damage or deterioration of the LAF sensor is detected based on the output of the air-fuel ratio (Japanese Unexamined Patent Application, First Publication, No. Hei 8-338290).
However, the sensor deterioration determination processing that determines the deterioration of the above-described LAF sensor must be carried out when the running conditions of the internal combustion engine satisfy predetermined conditions (hereinbelow, referred to as the monitor conditions). Therefore, even during the interval in which the sensor deterioration determination process is carried out, when the running conditions of the internal combustion engine do not satisfy the monitor conditions, the sensor deterioration determination processing must be temporarily suspended until the running conditions again satisfy the monitor conditions.
In particular, during the sensor deterioration determination processing, in the case that the air-fuel ratio must be controlled so as to be in proximity to a stoichiometric air-fuel ratio and the determination processing suspends and moves to a lean fuel mixture zone, the frequency of carrying out the deterioration determination processing of the sensor is reduced because of the time taken until the control of the stoichiometric air-fuel ratio is carried out again. Furthermore, output fluctuation during the switching between the lean fuel mixture and the stoichiometric air-fuel ratio occurs, and the drivability deteriorates.
The present invention solves the above-described problems, and has the object of providing an air-fuel ratio control apparatus that carries out efficient deterioration detection processing of the air-fuel ratio sensor.
Specifically, in the interval during which the deterioration determination processing of the above air-fuel ratio sensor is carried out, in the case that the running conditions of the internal combustion engine no longer satisfy the conditions allowing the carrying out of this deterioration detecting processing, switching to the lean fuel mixture is prohibited.
In order to attain the above object, the air-fuel ratio control apparatus according to the present invention is characterized in providing an air-fuel ratio detection device that is (the LAF sensor 17) provided in the exhaust system (the exhaust duct 16, the three way catalysts 19, 20, etc.) of an internal combustion engine, and outputs a signal approximately proportional to the concentration of oxygen in the exhaust, a lean fuel mixture zone determination device (in the embodiment, provided in the ECU 5) that determines whether or not the internal combustion engine is in the lean fuel mixture zone based on the running state of the internal combustion engine, a lean fuel mixture control device (in the embodiment, provided in the ECU 5) that controls the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine so as to be a leaner air-fuel ratio than the stoichiometeric air-fuel ratio when the internal combustion engine is determined to be in the lean fuel mixture zone by the lean fuel mixture zone determination device, a check zone determination device (in the embodiment, step S572 to step S584) that determines whether or not the internal combustion engine is the check zone, a malfunction check device (in the embodiment, step S521 to step S541) that checks for a malfunction of the air-fuel ratio detection device based on the output of the air-fuel ratio detection device, and a malfunction check termination device (in the embodiment, step S11 to step S17) that, during an malfunction check by the malfunction check device, terminates the malfunction check when the check zone determination device determines that the internal combustion engine is not in the check zone and at the same time prohibits this lean fuel mixture control for a predetermined time interval.